Conventionally, Japanese Patent No. 2513323 discloses a malfunction detecting apparatus detecting a seized-up state and/or a high-load application state of a motor used for the ABS control. More specifically, the seized-up state is the state in which the motor stops rotating because of the mechanical locking of the motor or a motor-driven pump.
In this malfunction detecting apparatus, as shown in the timing chart in FIG. 5, the motor is driven during the ABS control to operate the pump for sucking up and discharging of the fluid contained in a reservoir. Then, at the time of the termination of ABS control, the malfunction detecting apparatus detects abnormal conditions caused by seizing-up of the motor or an application of a high load. More specifically, a terminal voltage of the motor measured when the motor relay is switched OFF. At the time of the switching of the motor relay from the ON state to the OFF state, a counter electromotive force is generated in accordance with a rotating state of the motor. Then, a falling gradient of this counter electromotive force is measured. When the measured falling gradient is sharp, the malfunction detecting apparatus determines the abnormal conditions as described above.
Further, Published Japanese Translation of PCT Application JP-T-2002-506406 discloses the apparatus that repeatedly switches a motor relay ON and OFF in order to reduce noise produced by the passage of electric current through the motor for a long time. In this apparatus, as shown in the timing chart in FIG. 6, when a terminal voltage of the motor falls below a first threshold, a second threshold lower than the first threshold that is set for normal conditions is set in the motor speed control through which the motor relay is switched ON. When the apparatus determines that the terminal voltage of the motor falls below the second threshold, this determination denotes the detection of the application of a high load to the motor. Then, upon the detection of a high load, a motor-relay-ON duration is set to be longer than a duration of time before the high load application was detected. This setting renders the motor capable of rotating adequately even under a high load.
In the above Japanese Patent No. 2513323, an abnormal condition caused by seizing-up of the motor is detected. More specifically, when a counter-electromotive force in accordance with a rotating state of the motor has a sharp falling gradient, the apparatus detects an abnormal condition caused by seizing-up of the motor or a high load application. However, the seizing-up of the motor and the high load application are handled identically as an abnormal condition. Hence, it is impossible to detect the seizing-up of the motor and the high load application independently of each other. Further, the detection of an abnormal condition is made after the termination of the ABS control and, thus, it is impossible to detect an abnormal condition during the ABS control.
Further, also in the above Published Japanese Translation of PCT Application JP-T-2002-506406, after occurrence of the circumstances where the motor terminal voltage falls below the second threshold, the motor-relay-ON duration is made longer than that before the occurrence of such circumstances in all cases. After that, the motor-relay-ON duration is returned to that before it is extended. However, in either of these two abnormal conditions resulting from the seizing-up of the motor or the high load application, a terminal voltage of the motor comes to fall below the second threshold. Accordingly, it is impossible to detect the seizing-up of the motor and the high load application independently of each other.
Further, in the above Published Japanese Translation of PCT Application JP-T-2002-506406, as in the case of the application of a high load to the motor relay, when the motor has seized up, the motor-relay-ON duration is also extended temporarily, and then returned back.
At the time of the switching of the motor relay from the ON state to the OFF state, a high counter electromotive force is momentarily generated. Due to this high counter electromotive force, a semiconductor switch provided in the motor relay generates heat such as to damage the motor relay. Particularly, when the motor relay is repeatedly switched ON and OFF, a large counter electromotive force is generated every time the switching to ON from OFF is performed, which in turn increase heat generation. Finally, one more problem arising is the burnout of the motor relay.